There's Manchester Doki Doki in November (though I'm not sure about the date), there's the London MCM Expo in October (28th to the 30th), and coming up this weekend is the Japan Matsuri (also in London) on Sunday.

As previously mentioned, London MCM Expo is twice a year and the biggest organised event in the UK. MCM has two smaller events in Birmingham and Manchester, which, I personally haven't been too, but I haven't heard the best things about them, mainly since Manchester is brand new.

Other personal recommendations would be Kitacon, in Birmingham. It's better than Expo, which is more like an organised event for just buying stuff, whereas Kitacon and cons altogether have this mentality of everything there is more for the enjoyment of everyone.